The present invention is directed to vinyl polymerizable systems which when cured exhibit strength retention properties at elevated temperatures under moist conditions.
Composite materials are finding ever increasing utility in the structural engineering art. Emphasis is being placed on developing new composites and improved methods and resin systems for binding and holding the composite reinforcing materials together.
While epoxy resins are the most popular, three major deficiencies have become apparent with regard to the state-of-the-art epoxy resin systems for use as composite matrices.
First, is the high amount of resin flow during molding which can be as much as 50 percent of the original resin content. High resin flow may be attributed in some instances to the fact that some epoxy resins are liquid and flow during molding while, in other instances, the epoxy resins require a solvent carrier and retain the solvent during manufacture of the resinous article.
Second, is the requirement for use of autoclaves to provide the essential pressure necessary for the consolidation of resin bound complex parts. This requirement for pressure greatly limits the number of vendors capable of handling the epoxy resins.
Third, is their propensity to absorb moisture which is deleterious to the mechanical properties of high performance structural composites. It has been determined this results principally from absorption of moisture by the epoxy resin matrix. It has further been found that the presence of moisture has more of an influence on mechanical property degradation in glass fiber reinforced composites than for graphite fiber reinforced composites using the same epoxy resin system.
There are two modes of property degradation: initial reversible degradation resulting from plasticization of the epoxy resin matrix by the absorbed moisture and permanent degradation resulting from cracks in the epoxy resin matrix. The initial degradation can be predicted in neat epoxy resin castings based on standard diffusion models. However, the permanent degradation in composites cannot be predicted in any straightforward manner and is influenced by thermal peaks during exposure. A need has existed to provide resin systems for composite structures that do not undergo moisture degradation.
Ideally, such resin should provide the optimum moisture resistance; i.e., be hydrophobic. This feature must be achieved while maintaining suitability to processing by conventional technology and provide satisfactory interlaminal and flexural strength retention at elevated temperatures.
High vinyl content polybutadiene resins provide a base to induce hydrophobic properties. They exhibit, however, several deficiencies. The specific deficiencies are low adhesion to graphite and glass fibers, low elongation and a sharp drop in strength retention at moderately elevated temperatures of about 300.degree. F.
These deficiencies generally were countered by outstanding electrical properties and chemical resistance. Another advantage is that vinyl polybutadiene polymerization can be imparted at low resin flow; i.e., rapid gelation.
High vinyl content polybutadiene modified epoxy resins (hereinafter referred to as epoxy-polybutadiene copolymers) have been provided and are disclosed in U.S. Pat. No. 3,931,354, incorporated herein by reference. The epoxy-polybutadiene copolymer approach provides a compromise in which the excellent 350.degree. F. mechanical properties of the epoxy resins are combined with the hydrophobic character of the vinyl polybutadiene resin. The epoxy-polybutadiene copolymers generally provided acceptable dry strength retention up to about 350.degree. F. While hydrophobic characteristics are imparted by the polybutadiene moiety, such copolymer resin systems, although having a higher strength retention than conventional epoxy resins, were found to exhibit a drop in strength retention on exposure to moisture. Fiber reinforced composites formed from the high vinyl content polybutadiene modified epoxy resins exhibit an ultimate tensile strain to failure of less than about one-half percent. Preferably, a higher strain to failure percentage would provide extra toughness to the composite.
Resin systems having vinyl polybutadiene segments and which cure through vinyl groups on the polybutadiene exhibit some shrinkage upon curing which in the casting and molding arts is preferable minimized. Cross-linking of the vinyl polybutadiene segments is provided to a degree by aromatic bismaleimides. The aromatic bismaleimides, however, are not incorporated into the resin in a homogeneous fashion. This lack of homogeneity is occasioned by the aromatic bismaleimide not being readily soluble in the high vinyl content polybutadiene modified epoxy resins. Also, because of the relatively high melting point of the aromatic bismaleimides (generally greater than 300.degree. F.) they do not melt and mix into the resin prior to the onset of cure.
A resin system comprising copolymers of mixtures of 1,2 and 1,4 vinyl polybutadiene segments and certain aromatic bisimides, which resin upon heating to above 300.degree. F. cures to form bisimide cross-linking between vinyyl polybutadiene backbone structures, have been prepared. A resin system comprised of aromatic bisimide cross-linking between 1,2 and 1,4 vinyl polybutadiene segments is disclosed in co-pending United States Patent Application Ser. No. 877,127 entitled VINYL POLYBUTADIENE BISIMIDE COPOLYMERS, filed on Feb. 13, 1978 and assigned to the same assignee as this application. The vinyl polybutadiene bisimide copolymers provide acceptable dry strength and wet strength retention up to 350.degree. F. and higher. However, while exhibiting a high strength retention under moist conditions at elevated temperatures, the copolymers exhibit a toughness, as measured by the ultimate strain at failure for neat resin coatings, of less than about one percent. Such a strain at failure imparts a degree of brittleness to fiber reinforced composites formed from such copolymers. In addition, such vinyl polybutadienes exhibit shrinking during curing of the pendant vinyl groups. In casting and molding fiber reinforced composites from resins, it is desirable to minimize shrinkage of the resins.
In summary, while suited to low temperature performance, the resin systems of U.S. Pat. No. 3,931,354 fall short of desired retention of strength at elevated temperatures in the presence of moisture and the resin systems of United States Patent Application Ser. No. 877,127, while exhibiting better wet strength retention at elevated temperatures, fall short of desired toughness retention at elevated temperatures, are brittle and have undesired shrinkage levels.